This invention relates generally to rotary kilns and, more particularly, to reducing emissions including particulate matter in wet process rotary kilns.
Known wet processes include preparing a feed slurry containing up to 40% water. The feed slurry is a substantially homogenous mixture of water and ground material including limestone and clay. The feed slurry is pumped into a feed or cold end of the rotary kilns at a low velocity. Typically, the kilns are tilted at a ratio of approximately 1 foot vertical drop per 30 feet in the horizontal direction and rotate approximately 1 revolution per minute about an axis. The kilns are fitted with suspended chains that act as heat exchangers. The suspended chains are heated by the kiln flue gases. The slurry partially coats the chains as the kiln rotates and due to their large surface area, the chains act to evaporate water from the slurry. The chains also break up a resulting cake into a nodular dry material. The chains are laid out inside the kiln in a pattern extending between 100 to 250 feet. After the cake is broken into the nodular material, calcining and clinkering reactions take place in a calcination and a burning zone of the kiln. The clinker commences to cool down in a burner end of the kiln and discharges into a cooler, where it is cooled by ambient secondary air. The secondary air is preheated by direct contact with the cooling clinker. There is a large quantity of exhaust gasses that form an exhaust stream which flows counter current to the flow of slurry. The exhaust stream typically is at a velocity sufficient to pick up particles of dust and carry the particles out of the kiln. The gasses result from the fuel burned to supply heat as well as gasses released from the calcining and clinkering reactions plus gasses released from physical phase changes i.e., by boiling out of the slurry and nodular pellets.
However, the slurry typically contains trace amounts of organic materials and a conversion of the organic materials to dioxins can occur in an optimum temperature range within the chain section of a kiln or beyond. Accordingly the United States Environmental Protection Agency has proposed Maximum Achievable Control Technologies (MACT) standards regarding reducing emissions by reducing operating temperatures within rotary kilns.
A slurry of ground materials and water flows down a feed chute into a feed end of a kiln. A portion of the slurry in the feed chute is diverted by a flow control valve and is pumped through a pipe to a nozzle at a pressure sufficient to spray the slurry and coat a chain end and an inside diameter of a body of the kiln. The sprayed slurry creates a dust curtain that encapsulates particles of dust and rapidly cools the exhaust stream. Accordingly, the amount of dust and dioxins in the exhaust at the feed end is less with the nozzle spraying slurry than without the nozzle.